The present invention relates to the production of iron (III) sulfate and in particular to the production of preferred iron (III) sulfate solutions for use in water treatment and water purification facilities. In this specification, iron (III) sulfate refers to an iron salt with the iron in the ferric form, and iron (II) sulfate refers to an iron salt with the iron in the ferrous form or ferrous oxidation state.
It is well known that various hydroxides may be used as coagulating or floc producing agents in water purification and treatment facilities. The floc formed from such agents may be used to trap undissolved materials in the water such as organics, inorganic precipitates, and various biological matters. The floc precipitate generally effectively attracts and absorbs even very fine contaminating particles. During the treatment process, the floc grows in size, and ultimately is removed from the water by settling, skimming or filtering. Such agents may also be used in the treatment of water for the presence of phosphates, and as sludge conditioners in sewage treatment, whereby the sludge is rendered more capable of being filtered and treated.
The better known flocculating agents or floc producing agents are probably ferric hydroxide and aluminum hydroxide. In the past, aluminum hydroxide has often been preferred. However, recently the presence of aluminum hydroxides in treated water has caused some concern as a potential health and/or environmental risk. Thus, more recently, attention has been focusing on the use of ferric hydroxides as water purifying agents.
A well known process for the production of ferric hydroxide, in water treatment facilities, has been by the addition of ferric sulfate to the water to be treated. Generally, the alkaline content of the water rapidly induces hydroxide formation and ultimate floc production, after ferric sulfate is added to the water. If necessary, the pH, or hydrogen ion content of the water, may be adjusted for preferred hydroxide formation.
In the past, numerous problems have resulted from the utilization of ferric sulfate as the hydroxide producing agent. Generally, these can be traced to problems arising during the initial ferric sulfate production, and in the composition of the ferric sulfate agent used, rather than any inherent problem in the use of ferric sulfate itself. For example, ferric sulfate is generally produced from the oxidation of ferrous sulfate. If the oxidation does not go to completion, then some ferrous sulfate may still be present in the water treatment compound. While ferric sulfate is highly soluble in water, and rapidly forms relatively insoluble ferric hydroxide, ferrous sulfate forms ferrous hydroxide, which is somewhat soluble. Thus, in the past, the water being treated may have introduced therein, during the hydroxide forming step, the presence of soluble ferrous hydroxide that is not separated out by the floc. The ferrous hydroxide would then contaminate the treated water.
Further, ferrous sulfate is less soluble in water than is ferric sulfate. The presence of ferrous sulfate in the solutions to be used at water treatment facilities can cause problems, for example by precipitation from solution in the equipment of the plant.
Another problem with ferric sulfate compounds made according to prior processes has been the presence of excess acidity, generally free sulfuric acid, in the ferric sulfate product. Generally, when ferrous sulfate is oxidized, the resulting products are ferric oxide and ferric sulfate. However, if the oxidation takes place in the presence of sulfuric acid, the resulting product is primarily ferric sulfate, with relatively little of the oxide present. Generally, in past productions of ferric sulfate, according to such a general reaction scheme, excess acid would be present in the resulting ferric sulfate product. When such a product has been used in a water treatment facility, it has often been necessary to add alkaline compounds to the water, to adjust the pH of the solution. This not only can be costly, but also the temporary excess acidity, and the length of time taken for neutralization, may inhibit good floc formation. That is, generally, good floc formation has been found to be related to a relatively short period of time for hydroxide formation. If the length of time is increased, floc formation, in water purification, may not be as efficient. Further, the temporary excess acidity may be harmful to the pipes or machinery in the treatment plant. Also, the addition of alkaline materials to adjust pH may be undesirable under some circumstances.
Conventional methods of forming ferric sulfate generally result in a product which is a slurry, sludge or solid. Such products have, in the past, posed several problems. First, the material may be relatively difficult to handle or dissolve, especially if in the form of a rather solid cake. Further, impurities from the initial iron source may be present in the solid product, resulting in the addition of impurities to the water. Further, such a product may be relatively difficult to handle and package at the site of formation, especially since solid ferric sulfate may be relatively hygroscopic.